1. Field of the Invention
The present invention relates to a transmission output stage for a mobile telephone and, in particular, to a transmission stage for such a mobile telephone which is designed for two frequency bands.
2. Description of the Prior Art
At present, multiband mobile telephones are being developed and the first types are on the market which can operate at two operating frequencies, that is to say at 900 MHz and 1800 MHz in Europe in accordance with the relevant system definitions, and at 900 MHz and 1900 MHz frequencies in the USA.
First developments operate with narrow-band parallel amplifier chains in the transmit and receive section. Such dual-band output stages thus have, for each frequency band, an amplifier optimized for this band which is associated with a considerable expenditure on components and, therefore, with costs and space requirements. A dual-band output stage of this type having separate amplifier branches for each band is, for example, the TST0911 chip by the manufacturer TEMIC. When the chip is used in a mobile telephone, the two outputs of the chip are conducted to an antenna via a diplexer. Here, too, the disadvantages are the costs and the circuit board area needed and the necessary expenditure for filtering the harmonics.
To lower costs, the aim is to be able to process the two frequency bands with one amplifier chain. Thus, in the article by V. Güngerich, M. Pöbl: “Bessere Handys durch Gallium-Arsenid MMICs” [Better mobile telephones by using Gallium-Arsenid MMICs], Elektronik August 1998, p. 90-96, the structure of a CGY0819 amplifier chip by Siemens for multifrequency mobile telephones is described which exhibits separate RF inputs, the signals being conducted via a narrow-band amplifier in the respective band and the necessary output power being generated in a common output stage. In this arrangement, the preliminary stages can be switched on and off separately from one another depending on the operating state.
Since, therefore, the common output amplifier operates both for 900 MHz and for 1800 MHz, it is mandatorily of wide-band design. In 900 MHz operation, a strong first harmonic is, therefore, mandatorily produced at 1800 MHz. This harmonic can only be suppressed with additional filters which result in considerable insertion loss of the useful signal at the fundamental frequency and need which additional components. Furthermore, the matching elements must be elaborately switched with the aid of switches; for example, PIN diode switches or diplex filters at the output of the transmit transistor.
The present invention is, therefore, directed to a transmission output stage for a multifrequency mobile radio device which simplifies the critical switching of the output match of a dual-band transmission output stage for both frequency bands with simultaneous good suppression of the first harmonic of the lower-frequency signal.